avoid locking when message queue is empty in websocket

cocos/network/WebSocket.cpp

@@ -187,6 +187,12 @@ void WsThreadHelper::update(float dt)
 {
     WsMessage *msg = nullptr;
 
+	/* Avoid locking if, in most cases, the queue is empty. This could be a little faster.
+	size() is not thread-safe, it might return a strange value, but it should be OK in our scenario.
+	*/
+	if (0 == _UIWsMessageQueue->size()) 
+		return;	
+
     // Returns quickly if no message
     _UIWsMessageQueueMutex.lock();